Real-time estimation of tissue perforation risk during minimally invasive medical procedure

ABSTRACT

A method for performing a medical procedure, includes coupling a tip of a probe to tissue in an organ of a patient in order to apply the medical procedure using the probe. A force exerted by the tip on the tissue and a displacement of the tip created by the force are measured. A dependence of the force on the displacement is calculated. Based on the calculated dependence, a risk level of perforation of the tissue is estimated.

FIELD OF THE INVENTION

The present invention relates generally to invasive medical procedures,and particularly to methods and systems for estimating the risk oftissue perforation during application of medical procedures.

BACKGROUND OF THE INVENTION

Minimally-invasive intracardiac ablation is the treatment of choice forvarious types of arrhythmias. To perform such treatment, the physiciantypically inserts a catheter through the vascular system into the heart,brings the distal end of the catheter into contact with myocardialtissue in areas of abnormal electrical activity, and then energizes oneor more electrodes at or near the distal end in order to create tissuenecrosis.

A number of systems for probe-based medical procedures, such as, forexample, intracardiac ablation therapy, are commercially available, suchas the CARTO™ system offered by Biosense Webster Inc. (Diamond Bar,Calif.). CARTO tracks the position and operating parameters of thedistal end of the catheter and displays this information electronicallyon a three-dimensional (3D) anatomical map of the heart. CARTO enablesthe system operator to electronically tag locations that have beenablated on the map and thus to keep track of the progress of theprocedure.

To apply a catheter-based procedure, the physician typically forces thecatheter towards the heart inner surface tissue (myocardium). If duringthe medical procedure (e.g., ablation) the catheter exerts a force uponthe tissue that is higher than the force the tissue can tolerate, thecatheter may eventually perforate the tissue. As a result, blood orother fluids filling the heart chamber may flow through the perforatedtissue to fill up the space between the heart and the pericardium (i.e.,the pericardial cavity), a situation referred to as cardiac tamponade.

U.S. Pat. No. 6,351,667, whose disclosure is incorporated herein byreference, describes an apparatus for detecting pericardial effusionthat includes a measurement apparatus connected to a wire probe to beanchored on the right heart ventricle and to two other wire probes to beanchored in different regions of the pericardial sac. The measurementapparatus measures and displays the change in impedance between theindividual wire probes.

European Patent Application Publication EP 2248480, whose disclosure isincorporated herein by reference, describes various embodiments thatpredict the volume, area and/or depth of lesions created through the useof a force-time integration technique. Other embodiments control theenergy delivered to the ablation probe based on the contact forcebetween the ablation probe and the target tissue to prevent steampopping. In another aspect, various embodiments of the inventionreliably visualize the predicted volume, area and/or depth of lesionscreated during ablation procedures. One embodiment visualizes thepredicted lesions created utilizing a force contact density mappingprocedure. Another embodiment visualizes the predicted lesions throughthe use of a force-time integration technique. Yet another embodimentvisualizes the predicted lesions through the use of a force time andpower (and/or current) integration technique. Other embodiments predictthe occurrence and locations tissue damage such as perforation thatoccurred during the ablation process. Still other embodiments predictthe occurrence and location of isolation gaps that may occur during orafter the procedure.

European Patent EP 1229829, whose disclosure is incorporated herein byreference, describes a system for detecting the presence of aperforation in a body cavity. The system includes a fluid pressuresource; a medical device insertable into a body cavity, the medicaldevice fluidly coupled to the fluid pressure source for delivery offluid to the body cavity; and a pressure sensor positioned to detect apressure of the fluid delivered to the body cavity.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesa method for performing a medical procedure, including coupling a tip ofa probe to tissue in an organ of a patient in order to apply the medicalprocedure using the probe. A force exerted by the tip on the tissue anda displacement of the tip created by the force are measured. Adependence of the force on the displacement is calculated. Based on thecalculated dependence, a risk level of perforation of the tissue isestimated.

In some embodiments, calculating the dependence includes calculating agradient of the force as a function of the displacement, and estimatingthe risk level includes comparing the gradient to a predefined gradientthreshold. In other embodiments, estimating the risk level includespredicting that the perforation is not imminent upon detecting that thegradient is lower than the predefined gradient threshold. In yet otherembodiments, estimating the risk level includes predicting that theperforation is imminent upon detecting that the gradient is higher thanthe predefined gradient threshold.

In an embodiment, estimating the risk level includes identifying thatthe perforation has occurred upon detecting that the force isinversely-related to the displacement. In another embodiment, measuringthe displacement includes measuring a first position when the tip isinitially coupled to the tissue, measuring a second position when thetip exerts the force on the tissue, and calculating the displacement asthe difference between the second and the first positions. In yetanother embodiment, the method further includes indicating the estimatedrisk level to an operator of the medical procedure.

In some embodiments, indicating the risk level includes indicating anaudible or visual alert. In other embodiments, indicating the risk levelincludes outputting a first indication if the risk level indicates animminent perforation, and outputting a second indication, different fromthe first indication, if the risk level indicates an actual perforation.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for performing a medical procedure. Theapparatus includes an invasive probe and a processor. The invasive probecomprises a probe tip, which is configured to be coupled to tissue in anorgan of a patient. The processor is coupled to the probe and isconfigured to measure a force exerted by the tip on the tissue and adisplacement of the tip created by the force, to calculate a dependenceof the force on the displacement, and to estimate, based on thecalculated dependence, a risk level of perforation of the tissue.

There is additionally provided, in accordance with an embodiment of thepresent invention, a computer software product, including anon-transitory computer-readable medium in which program instructionsare stored, which instructions, when read by a processor that is coupledto a tip of an invasive probe for applying a medical procedure to tissuein an organ to which the tip is coupled, cause the processor to measurea force exerted by the tip on the tissue and a displacement of the tipcreated by the force, to calculate a dependence of the force on thedisplacement, and to estimate, based on the calculated dependence, arisk level of perforation of the tissue.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system for ablation that carriesout perforation risk estimation, in accordance with an embodiment of thepresent invention;

FIG. 2 is a schematic illustration of a probe performing cardiacablation, in accordance with an embodiment of the present invention;

FIG. 3 is a graph showing a relationship between force and displacementof an invasive probe, in accordance with an embodiment of the presentinvention; and

FIG. 4 is a flow chart that schematically illustrates a method forestimating the risk of tissue perforation, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

In minimally invasive procedures, the physician typically inserts acatheter through the vascular system into a body organ, and brings thedistal end of the catheter into contact with the internal tissue surfaceof the organ. In some cases, such as cardiac ablation, the medicalprocedure requires that the catheter would exert a certain level offorce or pressure on the tissue.

When the force exerted by the catheter is relatively small, tissueneighboring the catheter contact point is pushed away, forming atent-like shape. This effect is referred to as tenting. In tenting, thetissue maintains flexibility and can safely tolerate the catheter force.At higher force levels, there is risk of tissue perforation that shouldbe avoided.

Tissue perforation is of particular significance in medical proceduresthat cause the tissue under treatment to weaken. For example, duringcardiac ablation, energy is applied to the tissue to create localnecrosis. As a result, the structure or texture of the ablated tissuemay change and/or the tissue may become thinner or weaker, and thereforethe ablated tissue may not be able to tolerate the catheter force. Insuch medical procedures, there is a high risk of tissue perforation andaccompanying complications.

Embodiments of the present invention that are described herein provideimproved methods and systems for estimating the risk of tissueperforation during minimally invasive medical procedures. In an exampleembodiment, the catheter comprises sensors for performing real-timemeasurements of the catheter position (or displacement relative to someinitial position) and of the force the catheter exerts upon the tissue.Force and displacement measurements are delivered to a processor thatcalculates from successive measurements differential changes (e.g., ΔFand ΔD) to derive instantaneous gradient values ΔF/ΔD.

The magnitude and sign of the gradient are used for estimating the riskof tissue perforation. Experimentation has shown that as the catheter ispushed deeper into the tissue, the resistance of the tissue increases,and as a result a higher increase in the exerted force is required toachieve a constant increase in the displacement. In other words, theforce-displacement gradient increases at higher force and displacementlevels. Entry into a risk zone, in which perforation is imminent,corresponds to a force-displacement dependence that indicates a gradientthat is higher than a predefined threshold. If the dependence changessign, i.e., the force begins to decrease as a function of thedisplacement, perforation is likely to have occurred.

Thus, as long as the gradient is maintained positive and below apredefined risk threshold, the risk of imminent perforation is estimatedto be low. If the processor detects positive gradient values above therisk threshold, the risk of perforation is estimated to be high.Moreover, a situation in which gradient values become negative isindicative of a high risk that actual perforation has occurred.

In some embodiments, the force and displacement measurements may beused, in addition to or instead of the force-displacement gradient, forestimating the risk of tissue perforation.

In various embodiments, the processor produces suitable alerts to thephysician. Alerted by such real-time risk indications, the physician cantake suitable measures, in advance, to predict and prevent tissueperforation.

System Description

FIG. 1 is a schematic illustration of a system 20 for estimating therisk of tissue perforation during application of a minimally invasiveprocedure, in accordance with an embodiment of the present invention.System 20 comprises a probe 22, herein assumed to be a catheter, and acontrol console 24. In the embodiment described herein, it is assumed byway of example that probe 22 may be used for ablation of tissue in aheart 26 of a patient 28 in order to treat cardiac arrhythmias.Alternatively or additionally, probe 22 may be used for othertherapeutic and/or diagnostic purposes, such as for mapping electricalpotentials in the heart or in another body organ.

Console 24 comprises a processor 42, typically a general-purposecomputer, with suitable front end and interface circuits for receivingsignals from probe 22 and for controlling the other components of system20 described herein. Processor 42 may be programmed in software to carryout the functions that are used by the system, and the processor storesdata for the software in a memory 50. The software may be downloaded toconsole 24 in electronic form, over a network, for example, or it may beprovided on non-transitory tangible media, such as optical, magnetic orelectronic memory media. Alternatively, some or all of the functions ofprocessor 42 may be carried out by dedicated or programmable digitalhardware components.

An operator 30 inserts probe 22 through the vascular system of patient28 so that a distal end 32 (also in FIG. 2 below) of probe 22 enters achamber of heart 26. System 20 typically uses magnetic position sensingto determine position coordinates of the distal end inside heart 26. Inthis case console 24 comprises a driver circuit 34, which drivesmagnetic field generators 36 placed at known positions external topatient 28, e.g., below the patient's torso.

A magnetic field sensor 38 (also in FIG. 2 below) within the distal endof the probe generates electrical position signals in response to themagnetic fields from the coils, thereby enabling processor 42 todetermine the position, i.e., the location and typically also theorientation, of distal end 32 within the chamber. The magnetic fieldsensor, (i.e., the position sensor) typically comprises one or morecoils, usually three coils orthogonal to each other.

This method of position sensing is implemented, for example, in theCARTO™ system, produced by Biosense Webster Inc. (Diamond Bar, Calif.)and is described in detail in U.S. Pat. Nos. 5,391,199, 6,690,963,6,484,118, 6,239,724, 6,618,612 and 6,332,089, in PCT Patent PublicationWO 96/05768, and in U.S. Patent Application Publications 2002/0065455A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are allincorporated herein by reference.

In an alternative embodiment, the roles of position sensor 38 andmagnetic field generators 36 may be reversed. In other words, drivercircuit 34 may drive a magnetic field generator in distal end 32 togenerate one or more magnetic fields. The coils in generator 36 may beconfigured to sense the fields and generate signals indicative of theamplitudes of the components of these magnetic fields. Processor 42receives and processes these signals in order to determine the positionof distal end 32 within heart 26.

Although in the present example system 20 is assumed to measure theposition of distal end 32 using magnetic-based sensors, embodiments ofthe present invention may use other position tracking techniques, forexample, tracking systems based on impedance measurements.Impedance-based position tracking techniques are described, for example,in U.S. Pat. Nos. 5,983,126, 6,456,864 and 5,944,022, whose disclosuresare also incorporated herein by reference. Other position trackingtechniques, known to one having ordinary skill in the art, may be usedto determine the position of the distal end 32. Thus, in the presentapplication, the term position or displacement sensor is used to referto any element which provides signals, according to the location andorientation of a probe or a section of a probe, such as the probe'sdistal end, to console 24.

The distal end of probe 22 also comprises a force sensor 48 (also inFIG. 2 below) which is able to provide electrical force signals toprocessor 42 in order to measure the magnitude and direction of theforce on the distal end (or equivalently the force the catheter appliesupon the tissue). The direction of the force is typically measuredrelative to the symmetry axis of the distal end. Various techniques maybe used in measuring the force. Components and methods that may be usedfor this purpose are described, for example, in U.S. Patent ApplicationPublications 2009/0093806 and 2009/0138007, whose disclosures areincorporated herein by reference and which are assigned to the assigneeof the present patent application.

In order to ablate the tissue of heart 26, operator manipulates probe 22so that distal end 32 is at multiple locations on (or in close proximityto) the inner surface of the chamber. At each location, an electrode 40coupled to the distal end measures a certain physiological property(e.g., the local surface electrical potential). Processor 42 correlatesthe location measurements, derived from the position signals of sensor38, and the electrical potential measurements. Thus, the system collectsmultiple map points, with each map point comprising a coordinate on theinner chamber surface and a respective physiological propertymeasurement at this coordinate.

Processor 42 uses the coordinates of the map points to construct asimulated surface of the cardiac chamber in question. Processor 42 thencombines the electrical potential measurements of the map points withthe simulated surface to produce a map of the potentials overlaid on thesimulated surface. Processor 42 displays an image 44 of the map tooperator 30 on a display 46.

In the embodiments described herein, processor 42 uses at least theforce and displacement measurements performed by sensors 38 and 48 toassess the risk of tissue perforation. Processor 42 presents audiovisualindications and alerts regarding the estimated risk on display 46, toenable operator 30 to take suitable measures, in advance, to preventtissue perforation.

FIG. 2 is a schematic illustration of a probe causing tissue tenting, inaccordance with an embodiment of the present invention. The figuredepicts distal end 32 of a probe that is in contact with tissue surface100. By way of example, tissue surface 100 may represent the innersurface of a chamber wall in heart 26. The probe comprises positionsensor 38 and force sensor 48 as described above with reference to FIG.1.

In order to apply ablation, operator 30 typically forces the probetowards the tissue. Note that the probe may be positionedperpendicularly to the tissue, or obliquely as depicted in the figure.As a result of a perpendicular force component F 120, the tissue ispushed in the force direction into a tenting position depicted in FIG. 2in dashed lines. The position of the probe when it first comes intocontact with the tissue is marked by a line 108 and in the tentingposition by a line 124.

In an embodiment, position 108 serves as an initial or calibratedposition for displacement measurements. The displacement between initialposition 108 and position 124 is denoted D 128. Displacement D 128 isaligned with the direction of force F 120. Force sensor 48 and positionsensor 38 are configured to measure F and D, respectively.Alternatively, processor 42 may use raw (i.e., non-calibrated) positionand force magnitude/direction measurements to calculate D and F.

Various methods for measuring probe force and displacement are known inthe art, and any such method can be used to measure F and D. Forexample, U.S. Patent Application Publication 2012/0310116, whosedisclosure is incorporated herein by reference, describes a method thatincludes measuring a force exerted by a probe on tissue of a patient andmeasuring a displacement of the probe while measuring the force. Themethod further includes detecting a tenting of the tissue responsivelyto a relation between the measured force and the measured displacement.

As another example, U.S. patent application Ser. No. 13/680,496, filedNov. 19, 2012, whose disclosure is incorporated herein by reference,describes a method that includes pressing a distal end of a medicalprobe against a wall of a body cavity, and receiving from the probefirst measurements of a force exerted by the distal end on the wall. Themethod also includes receiving from the probe second measurementsindicating a displacement of the wall in response to the force. Themethod further includes estimating a thickness of the wall based on thefirst and the second measurements.

FIG. 3 is a graph 150 showing a relationship between force anddisplacement of an invasive probe, in accordance with an embodiment ofthe present invention. Graph 150 was created using a software simulationbased on physical analysis and field experience of the inventors. Thevertical force axis F and the horizontal displacement axis D mayrepresent, for example, F 120 and D 128 of FIG. 2. Relationship 150 isdepicted as a curve that is divided into three zones. In the range 0-D2,also referred to as a tenting zone 154, the flexibility of the tissue ismaintained and the relationship between the force exerted upon thetissue and the respective displacement indicates a positiveforce-displacement gradient. In the tenting zone, F and D exhibit anincreasing gradient behavior wherein the ratio between respective smallchanges in the force and displacement (i.e., the gradient or slope)ΔF1/ΔD1 at low force and displacement levels is smaller than the ratioΔF2/ΔD2, which is measured at higher force and displacement levels.

In a perforation-risk zone 158, which resides at the upper end oftenting zone 154, the gradient of curve 150 at points of displacementsin the range D1-D2 created by forces in the range F1-F2 is positive. Thegradient ΔF2/ΔD2, however, in the perforation-risk zone, issignificantly higher than in the 0-D1 zone. The behavior in zone 158demonstrates that the tissue resistance significantly increases due toforces higher than F1, and therefore a larger force increase is requiredat the perforation-risk zone than in the 0-D1 zone in order to createsimilar displacement increases.

Perforation zone 162 relates to forces lower than F2 and displacementshigher than D2 respectively. At the perforation zone the probe actuallypunctures the tissue and since the tissue no longer resists the forceapplied by the probe, force measurements drop rapidly and displacementmeasurements simultaneously increase. Note that in perforation zone 162the curve gradient ΔF3/ΔD3 would be negative.

FIG. 4 is a flow chart that schematically illustrates a method forestimating the risk of tissue perforation, in accordance with anembodiment of the present invention. Although the method is describedwith relation to cardiac ablation, the method is also applicable tonon-ablation procedures as explained below. The method is additionallyapplicable to body organs other than the heart. The method begins byoperator 30 inserting the catheter into heart 26 and bringing distal end32 in contact with the tissue at the site for ablation (or possiblyother probe-based treatment) at an initiation step 200. At this point,force and position sensors 48 and 38 start performing respectivemeasurements that are delivered to processor 42. Processor 42 resets ameasurement index i at an index reset step 204.

Processor 42 gets a force and displacement measurement pair F_(i) andD_(i) at a measuring step 208. Processor 42 increments index i and savesF_(i) and D_(i) in memory 50. Upon receiving new measurements, processor42 calculates an instantaneous slope or gradient value S_(i) at agradient calculation step 212. Processor 42 calculates a force changeΔF_(i)=F_(i)−F_(i−1) and a displacement change ΔD_(i)=D_(i)−D_(i−1)relative to the previous measurement index i−1. The processor calculatesthe instantaneous gradient S_(i)=ΔF_(i)/ΔD_(i) and saves S_(i) in memory50. Note that if either |ΔF_(i)| or |ΔD_(i)| is below a predefinedthreshold (e.g., |ΔD_(i)| is close or equal to zero), the instantaneousgradient result may be unreliable. In such cases the calculation ofS_(i) at step 212 is skipped and S_(i) may be discarded or copied fromS_(i−1). In some embodiments, processor 42 uses smoothed measurements byaveraging a predefined number of successive ΔF_(i) and ΔD_(i) samples.Note that averaging the difference samples ΔF_(i) between the indices iand i+M, is equivalent to taking the difference ΔF_(i+M)−ΔF_(i−1) (asimilar argument holds for the displacement differences). In yetalternative embodiments processor 42 averages the instantaneous gradientvalues S_(i).

Processor 42 then checks the sign of the gradient at a sign checkingstep 216. If S_(i)<0 perforation has presumably occurred and processor42 indicates a perforation alert to operator 30 at a perforationindication step 220. Otherwise, processor 42 checks whether the currentgradient S_(i) is greater than a threshold TH at a slope comparison step224. If S_(i)>TH, the force and displacement are assumed in theperforation-risk zone, and processor 42 alerts a perforation-riskindication at a perforation-risk indication step 228. The methodproceeds to check if the ablation is concluded at a termination checkstep 232.

Various methods can be used to detect conclusion of the probe-basedmedical procedure. For example, operator may decide that the medicalprocedure is completed for the current site by examining respectiveindications on display 46. Alternatively or additionally, processor 42can be configured to automatically decide if the medical procedure iscompleted. In embodiments in which probe 22 and processor 42 share acommunication channel (e.g., as in the CARTO system), operator 30 cansignal to processor via this communication channel of start and/or endevents regarding the medical procedure.

If at step 232 the ablation is not concluded the method loops back tostep 208 to collect subsequent measurements. Otherwise, ablation isconcluded and the method terminates.

The indications alerted to the physician at steps 220 and 228 above aretypically presented on display 46. For example, the processor may preseta possibly blinking warning text. Alternatively or additionally, theprocessor may activate a suitable audible sound to alert the operator.Further alternatively or additionally, the processor may activate anyother suitable indication to alert the operator of a perforation orperforation-risk situation.

Typically, the warning alert at step 220 would be different and morenoticeable than the alert indicated at step 228. The operator shouldrespond to a perforation alert by taking immediate suitable measures totreat the medical situation as known in the art.

Upon receiving a perforation-risk indication, operator 30 may take anysuitable action to prevent tissue perforation. For example, the operatorcan pull back the probe to reduce the force applied to the tissue.Alternatively or additionally, the operator may reduce or even shut downthe RF energy applied to the tissue (e.g., in ablation). Furtheralternatively, the operator can proceed to perform ablation at anothertissue site, and resume ablating the current site at a later occasion.In alternative embodiments, processor 42 automatically reduces or shutsdown the RF energy upon entering the perforation and/or perforation-riskzones.

The method described in FIG. 4 is an exemplary method, and othersuitable methods can also be used. For example, in addition to using thegradient value to estimate the perforation-risk level at step 224, forceand displacement values may also be considered, wherein higher valuesindicate higher risk of perforation. As another example, processor 42may use smoothed force and displacement measurements by applying amoving average window or any other smoothing method over multiple Sivalues at step 212.

In some embodiments, the method of FIG. 4 is used for probe-basedmedical procedures other than ablation and possibly in body organs otherthan the heart. In such procedures, applying a probe force upon theorgan tissue can still result in perforation to the organ tissue. Since,however, ablation typically weakens the tissue, when ablation is appliedthe instantaneous gradient values are expected to be lower, andtherefore the gradient threshold for detecting perforation orperforation-risk should be configured to a lower value relative tonon-ablation procedures.

Although the embodiments described herein mainly address estimating therisk of perforating the heart tissue during ablation, the methods andsystems described herein can also be used in other applications, such asin performing any medical procedure in which a probe may apply excessiveforce upon the tissue of an organ of the body.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and sub-combinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art. Documents incorporated by reference in the present patentapplication are to be considered an integral part of the applicationexcept that to the extent any terms are defined in these incorporateddocuments in a manner that conflicts with the definitions madeexplicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

The invention claimed is:
 1. An apparatus for performing a medicalprocedure, comprising: an invasive probe comprising a probe tip, whichis configured to be coupled to tissue in an organ of a patient; and anelectrode proximal to a distal end of the invasive probe; a magneticfield sensor within the distal end of the invasive probe, the magneticfield sensor configured to generate electrical position signals; and aprocessor, which is coupled to the invasive probe and is configured tomeasure a mechanical force exerted by the probe tip on the tissue and adisplacement of the probe tip created by the mechanical force, tocalculate a dependence of the mechanical force on the displacement, andto estimate, based on the calculated dependence, a risk level ofperforation of the tissue due to the mechanical force of the probe tipon the tissue, wherein the electrode is configured to measure localelectrical potential at a plurality of locations in the organ of thepatient to obtain a plurality of local electrical potentialmeasurements, wherein the processor is configured to correlate the localelectrical potential measurements, derived from the electrical positionsignals of the magnetic field sensor, with the local electricalpotential measurements to determine a plurality of map points, whereineach of the map points comprises a coordinate on an inner surface of theorgan and a respective local electrical potential measurement at thecoordinate, wherein the processor is configured to employ thecoordinates of the map points to construct a simulation of the innersurface of the organ, and wherein the processor is configured to combinethe local electrical potential measurements of the map points with thesimulation of the inner surface of the organ to produce a map ofelectrical potentials overlaid on the simulation of the inner surface ofthe organ, wherein the processor is configured to perform the steps of:setting a measurement index, receiving a first force measurement andfirst displacement measurement pair, incrementing the measurement indexand saving the first force measurement and first displacementmeasurement in a memory, upon receiving a second force measurement andsecond displacement measurement pair, calculating an instantaneousgradient value (Si), based on the change in force (AFi) and a change indisplacement (ADi), and saving the instantaneous gradient value (Si) inthe memory, wherein when either the change in force (AFi) or the changein displacement, (ADi) are below a predefined threshold, apreviously-calculated instantaneous gradient value (Si) is saved in thememory, and checking the instantaneous gradient value (Si), wherein whenthe instantaneous gradient value (Si) is negative, issuing an alert thatperforation has occurred and when the instantaneous gradient value (Si)is not negative, proceeding to a slope comparison step, the slopecomparison step comprising the step of determining whether theinstantaneous gradient value (Si) is greater than a gradient valuethreshold (TH), wherein when the instantaneous gradient value (Si) isgreater than a gradient value threshold (TH), the processor issues analert indicating a perforation risk, wherein the processor automaticallyissues instructions to reduce a supply of RF energy to the invasiveprobe when the instantaneous gradient value (Si) is negative, andwherein the processor automatically issues instructions to reduce thesupply of RF energy to the invasive probe when the instantaneousgradient value (Si) is greater than a gradient value threshold (TH). 2.The apparatus according to claim 1, wherein the processor is configuredto measure the displacement by measuring a first position when the probetip is initially coupled to the tissue, measuring a second position whenthe probe tip exerts the mechanical force on the tissue, and calculatingthe displacement as the difference between the second and the firstpositions.
 3. The apparatus according to claim 1, and wherein theprocessor is configured to indicate the risk level to an operator of themedical procedure.
 4. The apparatus according to claim 3, wherein theprocessor is configured to indicate the risk level by indicating anaudible or visual alert.
 5. The apparatus according to claim 3, whereinthe processor is configured to indicate the risk level by indicating afirst indication if the risk level indicates an imminent perforation,and indicating a second indication, different from the first indication,if the risk level indicates an actual perforation.
 6. A computersoftware product, comprising a non-transitory computer-readable mediumin which program instructions are stored, which instructions, when readby a processor that is coupled to a tip of an invasive probe when theprobe tip is coupled to tissue in an organ, cause the processor to:measure a mechanical force exerted by the probe tip on the tissue and adisplacement of the probe tip created by the mechanical force, tocalculate a dependence of the mechanical force on the displacement, andto estimate, based on the calculated dependence, a risk level ofperforation of the tissue due to the mechanical force of the probe tipagainst the tissue wherein the processor is configured to perform thesteps of: setting a measurement index, receiving a first forcemeasurement and first displacement measurement pair, incrementing themeasurement index and saving the first force measurement and firstdisplacement measurement in a memory, upon receiving a second forcemeasurement and second displacement measurement pair, calculating aninstantaneous gradient value (Si), based on the change in force (AF) anda change in displacement (ADO), and saving the instantaneous gradientvalue (Si) in the memory, wherein when either the change in force (AFi)or the change in displacement, (ADi) are below a predefined threshold, apreviously-calculated instantaneous gradient value (Si) is saved in thememory, and checking the instantaneous gradient value (Si), wherein whenthe instantaneous gradient value (S) is negative, issuing an alert thatperforation has occurred and when the instantaneous gradient value (Si)is not negative, proceeding to a slope comparison step, the slopecomparison step comprising the step of determining whether theinstantaneous gradient value (S) is greater than a gradient valuethreshold (TH), wherein when the instantaneous gradient value (S) isgreater than a gradient value threshold (TH), the processor issues analert indicating a perforation risk, wherein the processor automaticallyissues instructions to reduce a supply of RF energy to the invasiveprobe when the instantaneous gradient value (S) is negative, and whereinthe processor automatically issues instructions to reduce the supply ofRF energy to the invasive probe when the instantaneous gradient value(Si) is greater than a gradient value threshold (TH).
 7. An apparatusfor performing a medical procedure, comprising: an invasive probecomprising a probe tip which is configured to be coupled to tissue in anorgan of a patient; an electrode proximal to a distal end of theinvasive probe; a magnetic field sensor within the distal end of theinvasive probe, the magnetic field sensor configured to generateelectrical position signals; and a processor coupled to the invasiveprobe; a non-transitory computer-readable medium in which programinstructions comprising the computer software product according to claim6 are stored; wherein the computer software product, when read by theprocessor coupled to the probe tip of the invasive probe when theinvasive probe is coupled to the tissue, cause the processor to: (i)calculate a gradient of the mechanical force as a function of thedisplacement of the probe tip and the mechanical force exerted on theprobe tip, (ii) estimate the risk level by predicting that theperforation is imminent upon detecting that the gradient is higher thana predefined gradient threshold; and (iii) determine a perforation hasoccurred upon detecting that the mechanical force is inversely-relatedto the displacement after having previously determined that perforationis imminent at step (ii), wherein the electrode is configured to measurelocal electrical potential at a plurality of locations in the organ ofthe patient to obtain a plurality of local electrical potentialmeasurements, wherein the processor is configured to correlate the localelectrical potential measurements, derived from the electrical positionsignals of the magnetic field sensor, with the local electricalpotential measurements to determine a plurality of map points, whereineach of the map points comprises a coordinate on an inner surface of theorgan and a respective local electrical potential measurement at thecoordinate, wherein the processor is adapted to employ the coordinatesof the map points to construct a simulation of the inner surface of theorgan, and wherein the processor is adapted to combine the localelectrical potential measurements of the map points with the simulationof the inner surface of the organ to produce a map of electricalpotentials overlaid on the simulation of the inner surface of the organ,wherein the processor is configured to perform the steps of: setting ameasurement index, receiving a first force measurement and firstdisplacement measurement pair, incrementing the measurement index andsaving the first force measurement and first displacement measurement ina memory, upon receiving a second force measurement and seconddisplacement measurement pair, calculating an instantaneous gradientvalue (Si), based on the change in force (AF) and a change indisplacement (ADO), and saving the instantaneous gradient value (Si) inthe memory, wherein when either the change in force (AF) or the changein displacement, (ADO) are below a predefined threshold, apreviously-calculated instantaneous gradient value (Si) is saved in thememory, and checking the instantaneous gradient value (Si), wherein whenthe instantaneous gradient value (S) is negative, issuing an alert thatperforation has occurred and when the instantaneous gradient value (Si)is not negative, proceeding to a slope comparison step, the slopecomparison step comprising the step of determining whether theinstantaneous gradient value (Si) is greater than a gradient valuethreshold (TH), wherein when the instantaneous gradient value (S) isgreater than a gradient value threshold (TH), the processor issues analert indicating a perforation risk, wherein the processor automaticallyissues instructions to reduce a supply of RF energy to the invasiveprobe when the instantaneous gradient value (S) is negative, and whereinthe processor automatically issues instructions to reduce the supply ofRF energy to the invasive probe when the instantaneous gradient value(Si) is greater than a gradient value threshold (TH).